The invention relates to thermodynamic power production, and more particularly to binary vapor power plants and power production processes including both a superheated steam cycle and a second or "topping" cycle using another working medium permitting to achieve an inlet temperature higher than steam.
In conventional power plants, the working medium is water. However, the thermodynamic properties of water are such that no real increase in efficiency can be expected by increasing the superheat temperature. The heat sources which are available at present or which will be available in the near future can provide temperatures at which the theoretical efficiency of the cycle is increased very appreciably, but the theoretical increase cannot be exploited if water is used as coolant, inter alia because of the considerable irreversible flows of heat it causes in supercritical cycles.
More particularly, in contrast to light-water and CO.sub.2 and graphite reactors, which can produce only moderate temperatures, nuclear reactors now in service or under design (inter alia metal cooled fast neutron reactors, high-temperature gas reactors, liquid-salt reactors) can produce temperatures of up to 850.degree. C. or more. Similar considerations apply to the planned oxygen torch boilers. Later on, fusion will be another means of achieving very high temperatures.
In all such cases the use of a single cycle using steam as the working medium makes it impossible to take advantage of the high temperatures achieved.
The use of binary cycles has already been suggested. More particularly, a topping mercury cycle was added to the low-pressure steam cycle, mercury having the advantage of being liquid at ambient temperature and having a low saturating vapour pressure. On the other hand, it has disadvantages, inter alia its costs and toxicity.
A binary potassium-water cycle was also suggested; unfortunately, potassium has serious disadvantages; it is highly corrosive; it entails an upstream steam cycle associated with considerable irreversible phenomena; and it requires a very low absolute turbine exhaust pressure which is difficult to maintain at the normal turbine exhaust temperature.
Another prior art binary vapor power plant (U.S. Pat. No. 3,218,802 to David R. Sawle) includes a boiling sulphur nuclear reactor. The vaporized sulphur expands in a turbine, flows, through a stream superheating heat exchanger and is condensed in a heat sink; the condensed sulphur is then conveyed back to the reactor. Sulphur used as a working fluid in the topping cycle can be fed to the turbine as saturated vapour, thereby reducing the irreversible flows of heat. Due to the specific thermodynamic properties of sulphur, the vapour is superheated at the exhaust of the turbine, and the irreversible phenomena in the steam superheating heat exchanger are consequently decreased. The temperature and pressure at the exhaust of the sulphur vapour turbine are compatible with present day technology. Last, sulphur is less reactive than potassium and much less costy than mercury. It is not noxious.
On the other hand, sulphur has a detrimental effect on nickel at the temperature in the topping cycle and consequently corrodes those austenitic steels which have a high nickel content and the structural elements in contact with sulphur connot be made with such steels. However, the Cr-Mo alloyed steels have a poor resistance to creep above 550.degree. C. The Cr, Mn or Mo alloyed ferritic steels, even if surface coated, have a long term resistance to creep above 700.degree. C. which is unsufficient and they are not adapted for use in manufacturing exchange tubes subjected to a pressure differential in excess of 20 bars. In short, U.S. Pat. No. 3,218,802 does not provide any indication to the man of the art which would enable the latter to design a plant, particularly using fossil fuel. Last, the neutronic and thermal properties of sulphur are such as to render the development of a boiling sulphur power reactor at least doubtful.